1. Field of the Invention
The present invention concerns a temporary mechanical attachment device flexible in bending and designed to fracture at a predetermined tensile load, particularly but not exclusively suitable for temporarily attaching to a support an object intended to be ejected under precise kinematic and kinetic conditions.
It finds an advantageous but by no means exclusive application in spacecraft such as satellites in which a temporary attachment phase is followed by a separation phase, possibly with more or less forcible ejection. It can also find application in various terrestrial vehicles, maritime vehicles (surface vessels and submarines) and airborne vehicles (aircraft of all kinds in which a temporary connection is made that is to be released subsequently, possibly with immediate and precise movement of the object in question, for example to launch it with no external guidance.
A particular application of the invention is to the conducting of scientific experiments in the upper atmosphere of the Earth or to exploring the planets of the solar system using an automatic probe. It can be used, for example, to secure and then release or eject a storage vessel containing a chemical substance to be used as a tracer to study winds or magnetic fields: a storage vessel of this kind can be ejected from an artificial platform such as a sounding rocket or an interplanetary probe.
2. Description of the Prior Art
Various mechanisms are already known for providing temporary attachment of this kind followed by separation and more or less forcible ejection, in practice for achieving clean separation between the object and its support.
The following documents are particularly noteworthy in this respect: U.S. Pat. No. 4,554,905, WO-82/02527, U.S. Pat. Nos. 3,887,150, 2,888,294, 3,196,745, 3,597,919, 4,002,120, 4,187,759, FR-2,616,856 and FR-2,616,857.
The temporary attachment of the object to its support is in practice designed to be as stiff as possible, both in traction and in bending.
Temporary couplings relying on pyrotechnic separation are known: explosive bolts, explosive cutters and pyrotechnic release systems which retract an abutment member. Apart from the fact that they sometimes require a large number of parts, these various solutions always assume release commands separate from the ejection commands. Also, separation can disturb ejection and compromise its accuracy.
Also known are fragile couplings which are fractured by the ejection itself. This is the case with shear pins which are disposed transversely to the direction of ejection and anchored at their respective ends into the object and into the support. However, the shear pin technique has the following drawbacks.
It requires an extremely accurate fit between the moving parts; there is a risk of unwanted friction between the parts after the pin fractures; there is an additional part (the pin) whose two portions must be held in place after it fractures and which may scrape against the surface of the opposite other part; it is difficult to release the object, which is in practice retained by a number of pins, without unbalancing the system (through rotational (tilting) movement in particular).
This solution is complex because it is necessary to manufacture a number of parts and to assemble them accurately. Finally, it is not always reliable.
It has been found that a stiff coupling between the object and its support is not always advantageous and that, to enable ejection in any accurately defined direction with an accurately defined amplitude, it may be advantageous to make the coupling flexible but stiff in traction to enable the orientation of the object relative to the support before and after ejection to be defined only by the rod which applies the ejection thrust.
U.S. Pat. No. 5,136,925 in the name of the assignee hereof issued Aug. 11, 1992, discloses a simple and reliable temporary attachment device enabling relative movement of an object relative to its support in bending, which has a fragile area designed to fracture at a predetermined tensile load and advantageously adapted to center the object at least locally with reference to a possible ejection direction.
This patent discloses a device for temporarily attaching an object to a support having a body designed to be fixed to the support and including an annular end portion fixed to the object and joined to the body by a coupling. The coupling between the annular end portion of the receptacle fixed to the object and the body is flexible and fragile. An intermediate ring is joined to the annular end area and to the body by respective pairs of flexible axial tangs on respective diameters at 90.degree. to each other, the tangs of each pair being identical.
The central axial opening of the device enables sliding movement of the ejector rod, if any. Alternatively, it can enable the passage of electrical conductors (which are protected by it) or any other kind of connecting or transmission tube or cable that must be protected against relative tilting bending movement of the object and support.
According to preferred features of U.S. Pat. No. 5,136,925, the tangs of each pair have the same tensile strength and the tangs of one pair have a lower tensile strength than the tangs of the other pair. The tangs of each pair have the same tensile strength and tangs joining the intermediate ring to the annular end area have a lower tensile strength than the tangs joining the intermediate ring to the receptacle body. The tangs of each pair have the same characteristics in bending about the respective diameters, and the tangs of each pair have the same thickness transversely to the respective diameters and the same height parallel to the predetermined direction. The tangs of each pair have the same radial dimension and the tangs of one pair have a smaller radial dimension than the tangs of the other pair.
The intermediate ring and the tangs are an integral part of the body and include the annular end portion.
The body is hollow and contains an ejector rod having a rear portion designed to be subjected to ejection pressure and a front end designed to abut against the object.
The device further includes a spring member compressed axially between a flange inside the body and a bearing surface on the rear portion of the ejector rod to prestress the fragile coupling in traction; in particular, this prevents the attachment of the annular end portion to the object from loosening.
The annular end portion of the receptacle includes an externally screwthreaded portion cooperating with an internal screwthread formed at the mouth of the recess in the object, and the device also has a centering portion adapted to enter the recess with slight clearance.
An object of the present invention is to improve the performance of the temporary mechanical attachment device described in U.S. Pat. No. 5,136,925 by simplifying its construction and minimizing the risk of unwanted deterioration, notably in bending, but also and advantageously in torsion or even in shear, in service or at the time of ejection.
To be more precise, a first object of the invention is to provide a simple and reliable way of delimiting axial movement in bending (or even in torsion or, better still, in shear) of the device during handling, during storage or in service so as to protect the tangs from any excessive loading likely to degrade their fracture characteristics under tensile loading in particular or during possible ejection so as to limit relative movement in bending during such ejection to prevent any risk of binding/impact during movement of the piston within the device (for a piston 15 mm in diameter the sliding clearance in the attachment device may be only 0.5 mm). The invention is thus directed to improving the reliability of ejection, if any.
It has been found that in practice a small amount of relative movement in bending is sufficient to benefit from the flexibility in bending required in the aforementioned patent.
From the manufacturing point of view, another object of the invention, possibly in conjunction with the first mentioned object, is to reduce the precautions to be observed when machining the tangs and in particular to avoid the need for additional structures for stiffening the blank in bending during the machining of the tangs in the latter so as to minimize the risk of damage caused in particular by vibration during machining, so as to enable industrial scale manufacture at moderate cost.